The present invention broadly relates to a pressing shoe, and more particularly relates to a new and improved construction of a pressing shoe which is arranged to be pressed against the surface of a roller along a part of its circumference, the use of such a pressing shoe and a method of operating it.
Pressing shoes which can be pressed against the surface of a roller along a part of its circumference by at least one pressure chamber filled with hydraulic pressure medium, are used for example, as described in U.S. Pat. No. 3,974,026, granted Aug. 10, 1976, in the pressing part of a papermaking machine for dewatering a web of material, for example a web of paper, in which the web of material which is to be dewatered is passed together with a band which absorbs water, in a pressing gap between the pressing shoe and a rotating roller. Such pressing shoes may also be used for the support of the rollers in a rolling apparatus, as described in U.S. Pat. No. 4,041,752, granted Aug. 16, 1977.
In practice it is frequently necessary to be able to alter the amount of pressure applied by the pressing shoe, for example in order to be able to control the dewatering performance of a papermaking machine, as described in U.S. Pat. No. 4,074,624, granted Feb. 21, 1978, or in order to be able to regulate the lateral supporting force in a rolling apparatus, as disclosed for example in U.S. Pat. No. 4,149,397, granted Apr. 7, 1979. In order to control the amount of pressure applied, it has been proposed to alter the pressure of the hydraulic pressure medium in the pressure chamber by a suitable control or regulating arrangement. The range over which the amount of pressure applied can be controlled or regulated is, however, limited by the properties of the web of material which is to be pressed, or by the components which are used. The pressure of the pressure medium may not be increased to an unrestricted extent without exceeding the limit of stability of the materials which are to be pressed, e.g. of a web of paper, or of the components which are used, and without incurring the risk of damage to the material.
For certain applications, in particular for the dewatering of a web of paper, it is frequently desirable to be able to vary the amount of pressure applied and the duration of pressing independently of each other. The only free operational parameter which was available for this hitherto, namely the pressure, is not sufficient to satisfy these desiderata.
A change in the duration of pressing, with the speed remaining constant, has heretofore only been possible by changing the pressing shoe, for which purpose it is necessary to shut down the papermaking machine.